Vat dyes, particularly indigo, are conventionally utilized for dyeing textile fabrics, especially cotton fabrics, because of their fastness to washings, alkalis, mercerization and bleaching. Vat dyes may be divided into several groups according to their chemical nature and origin of which two of the most important are the indigoids and the anthraquinones. These vat dyes are marketed in the form of paste or powder and must be first reduced usually with sodium hydrosulfite, and applied to the textile fabrics and then oxidized to the original insoluble form. These dyes, particularly indigo, are expensive at first cost and in application, but are widely used because of their above-described excellent fastness properties.
Various attempts to recover these vat dyes from a vat dyeing operation for reuse in the vat dyeing operation to conserve or save on the costs of these vat dyes have been attempted. However, none of these prior attempts to recover vat dyes have been commercially successful for various and sundry reasons. Accordingly, no practical and commercially feasible process for recovery of vat dyes, particularly indigo, from textile dyeing operations for reuse in the dyeing operations currently exists.
Examples of dye recovery processes, including vat dyes, previously proposed and considered with respect to the present invention may be seen from the following prior art patents:
______________________________________ Patent Number Inventor Issue Date ______________________________________ U.S. 120,215 Sawyer October 24, 1871 U.S. 137,636 Thom et al April 8, 1873 U.S. 1,242,676 Forsyth October 9, 1917 U.S. 1,303,426 Wall May 13, 1919 U.S. 2,445,323 Galatioto July 20, 1948 U.S. 3,801,276 Gray April 2, 1974 U.S. 3,829,380 Oohara August 13, 1974 British 2156 Hugounenq September 2, 1864 British 2713 Auchinvole July 31, 1875 ______________________________________
In a different area beginning in about the 1930's, various experimentation and work has been performed with respect to the filtering of water utilizing the principles of reverse osmosis and the use of hyperfiltration and ultrafiltration devices consisting generally of a porous substrate having a dynamic osmotic membrane formed thereon.
By definition, osmosis is generally defined as the passage of liquids or gases through membranes separating solutions of different degrees of concentration by diffusion from the solution in which they are more concentrated to the solution in which they are less concentrated, provided the membrane is permeable to them. Osmotic pressure is generally defined as that pressure which develops when a pure solvent is separated from a solution by a semi-permeable membrane which allows only the solvent molecule to pass through it. The osmotic pressure of the solution is then the excess pressure which must be applied to the solution so as to prevent the passage into it of the solvent through the semi-permeable membrane. Accordingly, reverse osmosis occurs when pressure, in excess of the osmotic pressure, is applied to the more concentrated solution to cause the solution to pass through the membrane to the less concentrated side. The solutes or other particles in the more concentrated solution would be prohibited from passing through the membrane which is not permeable to such solutes.
The distinction between ultrafiltration and hyperfiltration devices is not clearly defined. However, both are generally classified as reverse osmosis devices utilizing semi-permeable membranes as molecular filters to be utilized as the separating agent and pressure as the driving force. In hyperfiltration and ultrafiltration devices, a feed solution is fed into the membrane unit, usually consisting of a substrate tube and a membrane formed thereon, where water and certain solutes pass through the membrane under an applied hydrostatic pressure. The solutes whose sizes are larger than the pore sizes of the membranes are retained and concentrated. The pore structure of the membrane thus acts as a molecular filter, passing some of the smaller solutes and retaining the larger solutes. The pore structure of this molecular filter is such that it does not become plugged because the solutes are rejected at the surface and do not penetrate the membrane. Apparently, the difference between hyperfiltration and ultrafiltration is in the size of the small molecular weight solutes rejected by the membrane and prevented from passing therethrough, with the hyperfiltration membranes rejecting or preventing passage therethrough of much smaller molecular size solutes than the ultrafiltration membranes.
Much of the earlier work utilizing reverse osmosis filtration was sponsored by the Office of Saline Water of the U.S. Department of Interior and related to efforts for effecting desalination of saline water to produce fresh water, as will be seen in the prior art patents listed immediately below.
More recently, this work with reverse osmosis filtration processes and apparatus spread into other areas including the textile industry where reverse osmosis filtration units have been experimented with for purifying waste water from dyeing and other types of textile finishing operations for reuse of substantially all of the waste water by filtering out or separating substantially all of the dissolved and undissolved impurities including the dyes and salts from the waste water. Representative examples of this prior work may be seen in the immediately below listed prior art publications.
One of the problems which surfaced with respect to utilizing reverse osmosis devices in the textile industry for the filtering of waste water from dyeing operations to substantially purify the waste water for reuse of the water is in the normally high pH of the waste water from these textile dyeing operations. The high pH of from about 9 to 13 causes rapid deterioration of the dynamically formed membranes utilized in the reverse osmosis filtration devices. The pH could be lowered by the addition of acid to the waste water; however, this is a costly treatment and would render the process unpractical from a commercial and economical standpoint.
Prior art considered with respect to the present invention and relating to processes and apparatus utilizing generally the principle of reverse osmosis filtration are, as follows:
______________________________________ Patent Number Inventor Issue Date ______________________________________ U.S. 1,825,631 Horvath September 29, 1931 U.S. 2,987,472 Kollsman June 6, 1961 U.S. 3,132,094 McKelvey, Jr. May 5, 1964 U.S. 3,331,772 Browncombe et al July 18, 1967 U.S. 3,396,103 Huntington August 6, 1968 U.S. 3,457,170 Havens July 22, 1969 U.S. 3,462,362 Kollsman August 19, 1969 U.S. 3,552,574 Lowe et al January 5, 1971 U.S. 3,625,885 Geinopolos et al December 7, 1971 U.S. 3,654,148 Bradley April 4, 1972 U.S. 3,758,405 Fremont September 11, 1973 U.S. 3,795,609 Hill et al March 5, 1974 U.S. 3,821,108 Manijikian June 28, 1974 U.S. 3,839,201 Miller October 1, 1974 U.S. 3,939,070 Roth February 17, 1976 ______________________________________